1. Field of Invention
The invention relates to a symbol timing recovery method used in DMT-VDSL and, in particular, to a method that uses the peaks and the company peaks in the frequency domain to calibrate the symbol timing.
2. Related Art
The very high data rate digital subscriber line (VDSL) utilizes the discrete multi-tone (DMT) technology. In comparison with the commonly seen asymmetric digital subscriber line (ADSL), the transmission speed of VDSL is higher (ADSL: 1.5-8M/0.3-2 Mbps 'VDSL: 2-26M/2-l6 Mbps) but the transmission distance of VDSL is shorter (ADSL: 4.8 km 'VDSL: 1.5 km). From the above-mentioned technical features, one sees that the ADSL technique is more suitable for an environment with users living over 1 km away from the telecommunication company. The users can enjoy the ADSL service using the existing phone lines. The VDSL technique is more suitable for an environment of community-type high-rise buildings. Fibers are connected from the telecommunication company to the high-rise building, where the community shares the bandwidth and the each individual user connects to the hub using the VDSL technique.
During data transmissions, the DMT-VDSL has to first encode the data and perform an inverse fast Fourier transform (IFFT). Before transmitting the converted signals, each discrete multi-tone signal is added with a cyclic extension (CE) to avoid inter-symbol interference (ISI), echoes, or cross-talks during transmissions. Once the receiving end receives the signals, it first removes the CE parts, performs a fast Fourier transform (FFT), and decodes to recover the original data. In this process, one has to accurately compute the boundary positions in order to remove the CE parts.
In the prior art, recovering the symbol timing in the time domain is achieved by utilizing the repeated data property of the CE's, where part of the rear data are copied to the front of the symbol and part of the front data to the end of the symbol after the IFFT.Therefore, when the receiving end gets a signal, the whole signal is delayed by N-samples, where N is equal to the DMT symbol length. Then, correlation is computed between the original signal and the delayed signal. The section with a high correlation is the boundary. However, the precision of this method is low in practice and it requires a lot of memory.
On the other hand, the method of recovering the symbol timing in the frequency domain is to extract a section of the received signal that is equal to the length of the DMT symbol arbitrary. Then, it is converted into the frequency domain using the FFT. The channel frequency response is computed in the frequency domain and converted back into the time domain to give the channel impulse response. Theoretically speaking, the channel impulse response has a peak response relative to the boundary of the DMT signal. Therefore, one can use this method to obtain the correct boundary position. The most commonly seen ADSL can use the above method to recover the symbol timing.
However, the VDSL is different from other transmission modes. During the VDSL training phase, the odd tone of the DMT symbol is used to transmit information to be exchanged between two VDSL transceivers. Therefore, when computing the channel frequency response, we cannot know all the band responses. This will result in two peak responses of equal height. In addition, when extracting a section of original symbols (including CE) and converting them into the frequency domain, if the extracted signal has two adjacent symbols both of them will generate peak responses. Combining the above two effects, one usually obtain four peak responses when using the conventional frequency domain symbol timing recovery method to compute the channel impulse response. Unfortunately, we cannot know which one is the correct peak response corresponding to the symbol boundary.
The method used in the ADSL technique cannot be directly applied to the VDSL technique. The U.S. Pat. No. 6,279,022 compares a known transmission frequency-domain pattern and the received frequency-domain symbol and computes to obtain the location of the peak response, thereby determining the boundary. It uses primarily the fact that the receiver in the ADSL transmission can obtain in advance all the symbols the transmitter will send. However, the there is no way to obtain all the symbols in advance in the VDSL system. Therefore, the method does not apply here.